Static pressure is the difference between the total pressure at a point and the air pressure that flows through an opening. The total pressure is equal to atmospheric pressure plus static pressure, but if you subtract static pressure from total pressure, you get what’s known as dynamic pressure or suction.
When you have a fan pulling air through an opening, you have negative static pressure at the exit of that opening. A fan creates a low-pressure zone behind it and pulls air into that low-pressure zone, creating negative static pressure. If there’s no fan pulling air through an opening, then there will be positive static pressure at the exit of that opening.
How Is Static Pressure in an HVAC Measured?
Static pressure measures the amount of resistance that air encounters when moving through an air duct system. It is measured in inches of water (in. wg), and the higher the number, the more resistance to airflow.
Ways to Measure Static Pressure
There are several ways to measure static pressure in an HVAC system. These include:
Pitot Tube Method
A pressure probe is placed at one end of the duct, and a manometer (a device used to measure pressure) at the other end. The difference between these two pressures can be used to approximate static pressure.
A manometer (a device used to measure pressure) is placed at one end of the duct, and a pressure probe at another end. The difference between these two pressures can be used to approximate static pressure.
Strain Gage Method
A strain gage sensor is placed on one side of an air duct and connected to a voltmeter on another side. Voltage readings increase as air flows through it and cause stress in the sensor, which increases its resistance and thus produces a voltage reading that can be converted into inches of water (in wg).
Factors That Determine Static Pressure in an HVAC
There are several factors to consider when determining static pressure in an HVAC system. These include:
System size is one of the most significant factors in determining static pressure. The larger the system, the higher the static pressure required. For example, if you have a 20-ton system and want to increase its capacity by 10%, it will require a lot more static pressure than a 10-ton system with the same 10% increase in capacity.
The climate where you live or work is a factor that determines static pressure in an HVAC, which is the amount of air that’s needed to move through the system.
If you live in a dry area, there’s less humidity in the air, so less water vapor is present to help with cooling and heating. That means that your HVAC system will have to work harder to move air around and need more pressure from fans and blowers.
If you live in a humid climate, then more moisture is present in the air, so there’s less need for added static pressure from fans and blowers.
Type of Insulation
The type of insulation in your building can significantly impact static pressure. If you’re using fiberglass or cellulose insulation, you’ll need to add an air-tight layer to the outside of your walls before installing a new HVAC system. This layer is made of aluminum foil and plastic sheeting, and it prevents any gaps between your walls and ceiling from letting air escape into surrounding areas.
If you don’t do this, your HVAC system could have too much static pressure and blow air all over the place—which isn’t suitable for anyone.
Type of Ductwork
If your ductwork is made of plastic, it’s more susceptible to static buildup than metal ductwork. This means that if you have plastic ductwork, it’s essential that you pay close attention to any changes in static pressure to adjust before there’s any damage done to your system or its components.
Size of Building
The size of the building is one of the most critical factors in determining static pressure. The larger the building, the more air needs to be moved around it. This means that more air will be pushed into smaller spaces and areas to reach other building parts, increasing static pressure.
Size of Ductwork
Your ductwork size in your HVAC system can also affect static pressure. The larger your ductwork is, the less resistance it will have when trying to move air through it. This means that less energy is needed to push air through large ducts than small ones, reducing static pressure.
What Happens if Static Pressure Is Too High or Too Low?
If you find that the static pressure is too high, it’s probably because you have a clogged filter. Try using a brush and some soapy water to get it clean. If that doesn’t work, try removing the filter and cleaning it with a paper towel. If that still doesn’t work, contact the manufacturer for more specific instructions.
If you find that your static pressure is too low, it’s probably because something is blocking one of the filters in your system. Remove them and clean them with a paper towel to see if they’re clogged up. If they are blocked up, you may need to replace them or call in an expert to help you fix this problem.